Vitamin D Trials for Cancer Prevention

The randomized controlled trial evidence on vitamin D supplementation and cancer prevention has been mixed to date. The Women’s Health Initiative trial, which followed roughly 36,000 women for an average of seven years, failed to find any reduction in colon or breast cancer risk in women who received daily supplements of 400 IU of vitamin D and 1,000 mg of calcium, compared to those who received a placebo. (1, 2) But that study had several limitations, chief among them the relatively low dose of vitamin D. (3, 4) Also, some people in the placebo group decided on their own to take extra calcium and vitamin D supplements, minimizing the differences between the placebo group and the supplement group, and about one third of the women assigned to vitamin D did not take their supplements. (5)

A more recent trial among nearly 1,200 postmenopausal women found significant reductions in overall cancer incidence among those randomized to receive 1,100 IU of vitamin D plus 1400-1500 mg calcium. (6) The Vitamin D and Omega 3 Trial (VITAL) study, which is testing 2,000 IU of vitamin D per day, should offer more answers on the role of vitamin D in cancer prevention, although it could be affected by some of the same limitations as the Women’s Health Initiative.

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